Seismic Observation Networks in Japan.

Abstract

More than five hundred permanent stations are included in seismic observation networks in Japan. Those observation stations belong to a numbers of networks; the nation-wide network operated by the Japan Meteorological Agency (JMA), and subregional to local networks operated by national universities and other institutions, including the National Research Institute for Earth Science and Disaster Prevention (NIED). Most of the non-JMA networks were established under the national program for earthquake prediction, which began in 1965. The JMA network has history of more than 100 years, and it now includes about 190 stations which are equipped with both short- and/or intermediate-period seismometers. The other networks, for the most part, are instrumented by highly sensitive short-period seismometers typically with a natural period of 1.0s. Almost all of the networks are linked by digital telemetry, and are supported by a computer-aided data processing systems which execute seismogram reading and hypocenter determination automatically or semi-automatically. The networks of national universities are basically independent of one another, but they carry out on-line and off-line exchange of observation data to some extent. Currently, the earthquakes occurring in Japan are uniformly located down to magnitude 3.0 except for the Ryukyu region by the JMA network, and down to magnitude 2.0 except for northern Hokkaido and Kyushu-Ryukyu arc by integrating data from the university networks. A total of about ninety thousand hypocenters per year are located by Japanese seismic networks.